A suitable device is needed for the direct measurement of oxygen partial pressure (pO2) in blood and tissue for both clinical and research applications. Methods currently available for measuring pO2 lack convenience, reliability, speed, and applicability to many situations of interest. Efforts to develop electrical sensors have not been successful. It is desirable to have a very small pO2 sensor which can be inserted into a blood vessel or tissue with little disturbance, that will provide instantaneous pO2 monitoring for either short or extended periods of time. A fiber-optic sensor is ideal for this application, with the advantage, for physiological use, of very small size and flexibility, safety, and low cost. A pO2 sensor has been developed, based upon the principle of fluorescence quenching by oxygen. The feasibility of the sensor and its satisfactory performance have been demonstrated in in vitro and in vivo tests in preceding years. The development of the sensor as a needle probe has been accomplished.